Chinese officials have bowed to pressure and pledged to investigate the strange death of a Tiananmen protest veteran after widespread suspicion and anger, Hong Kong media have reported. Li Wangyang, who spent 20 years in prison due to his role in the 1989 pro-democracy protests, was found dead in hospital last week in central Hunan province with a noose around his neck.

Officials in Shaoyang originally said that Li Wangyang took his own life, but later described his death as an accident. Relatives and friends said his feet were still touching the floor, questioned whether he was physically capable of hanging himself given his severe disabilities after his lengthy and gruelling jail terms, and said he had been making plans for his future.

"In addition to commissioning authoritative forensic experts from outside the province to carry out a postmortem, we have launched a further investigation by experienced criminal investigation experts," a Hunan police spokesman said, according to the Beijing-backed Hong Kong China News Agency.

He said the decision was made because of the attention Li's death had roused, including in the foreign media, and said the results of the investigation would be announced promptly.

Activists say Li's sister and brother-in-law have been detained and they believe his body has already been cremated.

Yin Zhengan, a close friend of Li, told the South China Morning Post: "[The investigation] is more of a gesture for provincial police authorities to get involved.

"I can hardly see positive prospects for a convincing result because the most telling evidence has been destroyed with the cremation of Li's body."

Around 12,000 people in China and overseas have signed a petition demanding an investigation into Li's death. Thousands took to the streets in protest in Hong Kong.

Even senior officials in Hong Kong have questioned the death, with the food and health secretary, York Chow, saying he doubted whether Li was able to hang himself, while outgoing chief executive Donald Tsang described it as "suspicious" and said he understood the views of the Hong Kong people, in a rare comment on human rights issues.